1. Field of the Invention
The present invention relates generally to a deceleration control system for supplying additional air or an additional air-fuel mixture to an engine during deceleration of the engine and particularly to a deceleration control system of this type which is rendered operative to supply the additional air or the additional air fuel mixture to the engine only when the temperature of a catalyst of a catalytic converter is undesirably increased above an allowable limit.
2. Description of the Prior Art
As is well known in the art, when an engine is in its decelerating condition, since a throttle valve is fully closed notwithstanding that the engine speed is high, the vacuum in an intake passageway downstream of the throttle valve is extremely increased, that is, the amount of air drawn into the engine is extremely insufficient. As a result, the combustion of an air-fuel mixture in the engine is deteriorated and, in an extreme instance, becomes impossible. This results in the production of engine exhaust gas containing a large quantity of burnable components such as hydrocarbons (HC).
On the other hand, the engine is provided in its exhaust system with a catalytic coverter which efficiently oxidizes the burnable component contained in the engine exhaust gas in the presence of secondary air fed into the engine exhaust gas by a secondary air supply device employing an air pump or a vacuum induced in an engine exhaust gas passageway. In an engine employing a lean air-fuel mixture, excessive oxygen contained in engine exhaust gas is used for oxidization of the burnable component of the engine exhaust gas in a catalytic converter and can be substituted for a secondary air supply device.
However, when the engine misfires to discharge exhaust gas containing a large quantity of burnable component such as hydrocarbon with secondary air fed into the engine exhaust gas or a lean air-fuel mixture fed into the engine, all the burnable component in the engine exhaust gas is often burned at a stretch in the catalytic converter. In such a condition, the temperature of the catalyst in the catalytic converter is abnormally increased which results in deterioration of the catalyst and could also induce burning of the catalyst in an extreme case.
As a solution to such a problem, an engine is provided with a deceleration control system comprising an additional passageway bypassing a throttle valve and a control valve for opening the additional passageway during deceleration of the engine, or means for opening the throttle valve a small amount during deceleration of the engine. The deceleration control system serves to supply the engine with additional air or an additional air-fuel mixture to thereby compensate deficiency of air drawn into the engine and prevent a vacuum in an intake passageway downstream of the throttle valve from being extremely increased.
However, a conventional deceleration control system has been continually rendered operative during deceleration of an engine to supply the engine with additional air or an additional air-fuel mixture even when the temperature of a catalyst of a catalytic converter is below an allowable limit below which there is no risk of the catalyst being deteriorated by heat. This results in an increase in fuel consumption and a lowering of engine braking effect or performance.